ELECTRONIC DESIGN NOTES - CAPACITORS

The electronics world is ruled by the
MOS [Metal Oxide Semiconductor] technology. That is, tiny pieces of silicon (or selenium/germanium), thin layers
of metal (aluminum or copper) and even thinner layers of metal oxide (aluminum oxide or copper oxide).

1. TYPES OF
CAPACITORS
In electronics, capacitors are used in principal to filter spikes and
abrupt voltage variations;
however, capacitors are good for many other things. Let's summarize what we can do with capacitors on our PCBs:

1. to filter EMI (induced Electro-Magnetic Interference);2. to control the amount of reactance or inductance in AC circuits;3. to build RC/LC timing circuits;4. to control the "slew-rate" (the ramp) of electrical pulses;5. to build resonant circuits;6. to build frequency filtering circuits;7. to build analog-to-decimal circuits;8. to achieve coupling inductance;9. to build SMPS (Switched-Mode Power Supply) power-pumps;10. to store energy;11. many more.

The most general classification groups capacitors as being:

1. fixed
2. variable

Considering the way they are inserted in electrical/electronic circuits, capacitors are:

1. through hole
2. surface mount
3. big capacitors, or batteries of capacitors having specially designed mechanical fixtures and connectors

A capacitor is made of two metallic plates positioned at a certain distance, and having a dielectric material in between.
The nature of the dielectric influences capacitors' general construction.
Therefore, considering their dielectric capacitors can be:

NOTEAlthough they are used a lot in DC circuits, please note that
capacitors are AC-transient electrical components. As mentioned in
Design Notes 1, there are no instances of pure DC circuits; all of them have some AC characteristics.

2. USEFUL FORMULAS
In order to work with capacitors we need few mathematical formulas (as tools) close at hand. Here they are:

NOTEIn case you are unfamiliar, the notations within square brackets represent the unit. For example,
Xc [Ω]
means: Xc is measured in ohms. That is an important aspect, because we could have instances when
the measurement unit is in mili-ohms, Xc [mΩ], or
kilo-ohms, Xc [kΩ], therefore the formulas we use
need to be scaled appropriately. For
that reason the measurement unit must be added explicitly within square brackets.

4. GROWTH AND DECAY IN RC CIRCUITS
A capacitor in series with a resistor forms a timing circuit. This function is used
a lot in
electronics.

In RC
circuits the time constant "T" is:
T [s] = R [Ω] * C [F]

In order to charge the capacitor to full capacity, however, it takes roughly 5 time constants (R*C) calculated
with the above formula. The "decay curve" behaves perfectly similar to the "growth" one
illustrated further down, having only an inverse second derivate (the curve holds water).